The create your own roster if you are very senior, is a function of the JSS bidding system.
If you are the most senior FO on a fleet, and you place an exceedingly specific bid for the work you want in a month, and the sequence of work you have asked for is legal and builds you a complete month’s work - you will get it.
If you are second highest seniority and you do the same, you will get it if the more senior pilot didn’t already take those trips for their roster. This goes on as JSS works its way down the list of bids. The further down the list you go, you can still try for your specific bid for work, but you are less likely to get it as it’s more likely more senior people will already have taken that work, so you have to bid realistically because JSS needs to build you a complete roster and if it needs to start ignoring what you asked for - it will.
The above is a slightly simplistic description of how it works, but explains the ‘create your own roster’ phrase you have heard.
Note. You can be super senior FO on a fleet, getting great rosters, then do a command on type and suddenly find yourself very junior. In this situation, your success with roster construction will plummet. The same can happen happen with an aircraft change. It purely depends on your relative seniority wherever you are.